1. Field of the Invention
The invention relates to the field of heating elements for industrial electrical furnace installations, and in particular to a heat conductor support disk for use in a heating element.
2. Background Information
So-called heating plugs used in industrial furnaces are constructed with the help of several ceramic disks. The purpose of the ceramic disks is to support and position heating resistor elements or heat conductors which are inserted through apertures in the disks.
In such disks, a center aperture for a supporting element is located on a symmetrical axis of each disk, and apertures for heat conductors are provided uniformly distributed on at least one circle coaxial with a respective disk center.
In this type of heating plug, the ceramic disks are spaced apart and arranged parallel to one another, with the supporting element, which is usually a rod or tube, being inserted through the center apertures of the ceramic disks. This disk and supporting element configuration is usually pushed into a protective tube generally made of either metal or a ceramic material. The protective tube is also known as a radiant tube because during operation it emits heat toward the exterior.
Due to thermal deformation effects of the components, there must be a certain play between the exterior circumference of the heating plug and the interior diameter of the protective tube. However, the heating element may also be built into a thermic installation as a freely radiating and operating heating element.
The exterior circumference of these ceramic disks is usually circular, and all the apertures of the ceramic disks have diameters which remain the same over their entire length.
In order for the heating elements to function, it is, among other things, essential for the heat conductors to be constantly supplied with oxygen. This is because the heat conductors and the other metal members of the heating element comprise an aluminum-containing alloy, particularly if a higher range of temperatures is to be obtained. During operation, the oxygen is combined with the aluminum in the alloy and forms Al.sub.2 O.sub.3. The formation of this oxide (ceramic) is desired because it essentially increases the service life of the heating element. The described phenomena are known.
In prior art heating elements, the metal members of the heating elements are supplied with oxygen but generally not in sufficient quantities. The reason for this is that each of the ceramic disk forms a barrier for the oxygen (air) which is to flow through the heating element, all apertures in the ceramic disk being almost completely filled with heat conductors and the supporting element. Also, at the outer circumference of a ceramic disk, there is only a very narrow clearance (slit) between the exterior circumference of the disk and the protective tube, whose size additionally depends on the respective temperature.
These types of heating elements are frequently provided with thermoelements. These thermoelements are rod shaped and may extend over the length of the heating element. In this case, it is known to provide one sole recess at the circumference of each ceramic disk and then to place the thermoelement into the mutually aligned recesses. However, the installation in this case is very cumbersome and expensive, because attention must always be paid that the mentioned recesses are aligned with one another.
It is a further drawback of the described prior art construction that the heat conductors make contact with the ceramic disks via a circular region over a considerable distance due to the bores (apertures) which accommodate the heat conductors. As mentioned earlier, the diameter of these bores is uniform over their entire length. Since the ceramic disks are poor heat conductors, heat accumulates in this region, thereby not only causing deterioration in the degree of effectiveness of the heating elements, but particularly causing the ceramic disks to act on a relatively wide circular region on each heat conductor, causing the latter to be damaged in the process. This is the result of thermal deformations, which are often significant.
The above-described prior art has been previously employed by the applicants. However, the applicants are also aware of modifications used by other firms which are described in more detail below. However, these modifications of multi-aperture disks as heat conductor supports are apparently not intended for industrial furnace construction, but instead, for use at relatively low temperatures.
In one prior art modification of a multi-aperture disk, recesses, which are approximately semi-circular having circular edges, are evenly distributed over the entire circumference of the disk. These recesses are spaced apart with respect to one another such that a portion of the original disk edge (circular) remains between each respective recess.
However, this has the drawback that the transition between the approximately semi-circular recesses and the original circular disk are relatively sharp edged. These sharp edges can consequently break off during operation and, particularly if the heating element is arranged horizontally, the sharp edges act on the interior surface of the protective tube and may destroy the oxide layer there.
In the case where the heating element is arranged horizontally, two things can happen: (1) the ceramic disks make contact with the portion of their exterior circumference remaining on the interior surface of the protective tube resulting in a disadvantageous single-point contact, since there must be a certain play between the exterior diameter; or (2) the disks rest with two adjacent edges on the interior side of the protective tube. Although this defines the contact, it occurs at the expense of possibly damaging the interior wall of the protective tube caused by the action of the sharp edges.
In the modification already mentioned, the apertures supporting the heat conductors are chamfered, apparently in order to prevent damage to the ceramic material during the assembly process of pulling the ceramic disks onto the heat conductors. In a prior art modification, the chamfering depth is 2 mm if the total length of the bore is 11.2 mm. In another modification, the chamfering depth is approximately 4 mm for a total bore length of approximately 12 mm.
Another known modification has the mentioned recesses at the edge of the ceramic disk having smoothly penetrating bores, while two other modifications are provided with the mentioned recesses at the circumference of the ceramic disk. Other modifications, again, show ceramic disks without the mentioned recesses and without the chamfering.
There is another available type of multi-aperture ceramic disk some variations of which also have the previously mentioned chamfering on the order of magnitude of 2.times.1 mm for a 15 mm total length of bore. However, other disks of this type do not have chamfering, and none of these ceramic disks have recesses at the edge of the disk. Here chamfering also apparently merely serves to protect the edges of the bores.
A further prior art modification involves this type of ceramic disk without recesses at the edge of the disk, but with two circular apertures. The outer aperture has circular aperture chamfering. In one variation, the aperture extends the entire thickness of the disk, the entire length of the bores being 16 mm and the chamfering being 2.times.5 mm long. In a second variation of this type of disk, the disk is 19 mm thick and the chamfering is also 2.times.5 mm long. Although in the first disk, the chamfering scarcely extends over half of the entire length of the bores, the second disk indicates that there too, attention was only focused on edge protection.
It should be further pointed out that in all known prior art multi-aperture ceramic disks, a transition between an inner (central) portion of the bore and the chamfering is edged. This has the disadvantage that the circular edges may damage the surface of the heat conductor to an increased extent.